1. Field of the Invention
The present invention relates to skin treatment compositions and especially to emollient compositions. More particularly, the present invention relates to compositions comprising lanolin and lanolin acid esters.
2. Description of the Prior Art
Under normal conditions, the water content and vapor pressure of the epidermis are higher than the surrounding air and water evaporates from the surface of the skin. Skin becomes dry due to excessive loss of water from the stratum corneum when exposed to low relative humidities, insufficient hydration from the lower epidermal layers and/or air movement.
Frazier, et al., A Formulary For External Therapy of the Skin, Charles C. Thomas, Springfield, Ill. (1954), pp. 55-69, defined an emollient material as one which prevents or relieves dryness of the skin. Strianse, S. J., "Hand Creams and Lotions" in Sagarin, E., Cosmetics: Science and Technology, Wiley-Interscience, New York (1972), p. 180, defined an emollient as an "agent which, when applied to a dry or inflexible corneum, will effect a softening of that tissue by inducing rehydration".
Skin dryness, and reduced flexibility, of the stratum corneum cannot be corrected by the addition of oily materials, but the skin will become flexible when rehydrated, even in the absence of oily, materials.
It is the decrease in the water content of the stratum corneum which is the main cause of the dry feeling in chapped skin. Inasmuch as the water which diffuses from the dermis to the upper layers is limited in quantity and insufficient to balance the water lost from the skin surface by evaporation, especially in low relative humidities, the skin approaches a certain degree of inflexibility and stiffness. It is here that an emollient finds its maximum use, in serving as an occlusive agent, i.e. a hydrophobic material which reduces or prevents passage of water into or through a film of these substances. The occlusive material acts as a barrier to evaporation of water from the skin surface, and hence permits rehydration of the corneum. In addition, the emollient imparts slip but does not necessarily soften the skin because of its "lubricating" properties.
Barnett, G., "Emollient Creams and Lotions" in Sagarin, E., Cosmetics: Science and Technology, Wiley-Interscience, New York (1972), pp. 34-36, indicates that emollients comprise a long list of materials which may be classified in the following types:
(1) Hydrocarbon oils and waxes; PA1 (2) Silicone oils; PA1 (3) Triglyceride esters; PA1 (4) Acetoglyceride esters; PA1 (5) Ethoxylated glyceride; PA1 (6) Alkyl esters; PA1 (7) Alkenyl esters; PA1 (8) Fatty acids; PA1 (9) Fatty alcohols; PA1 (10)Fatty alcohol ethers; PA1 (11) Ether-esters; PA1 (12) Lanolin and derivatives; PA1 (13) Polyhydric alcohols (polyols) and polyether derivatives; PA1 (14) Polyhydric alcohol (polyol) esters; PA1 (15) Wax esters; PA1 (16) Beeswax derivatives; PA1 (17) Vegetable waxes; PA1 (18) Phospholipids; PA1 (19) Sterols; and PA1 (20) Amides. PA1 (A) from about 50 to about 95% by weight of lanolin; PA1 (B) from about 5 to about 50% by weight of at least one ester of a lanolin acid; and PA1 (C) from 0 to about 25% by weight of lanolin oil.
Lanolin is the unctuous secretion of the sebaceous glands of sheep which is deposited onto the wool fibers. It softens the fleece and serves to protect the fleece against the elements. It is a wax, not a fat. It is a complex mixture of esters, diesters and hydroxy esters of high molecular weight lanolin alcohols (69 aliphatic alcohols (C.sub.12 -C.sub.36) and 6 sterols have been identified in lanolin) and high molecular weight lanolin acids (approximately 138 acids (C.sub.7 -C.sub.41) have been identified in lanolin). Lanolin is a by-product of the wool-scouring industry.
Wool grease constitutes 10-15% of the weight of sheared wool, depending on the breed of sheep, anatomical area sheared, inner or outer fleece, and season. The average composition of Australian fleeces is 11-16% grease, 6-8% suint (potassium salts of various organic and inorganic acids in the sweat), 10-12% water, 8-19% dirt and 49-61% wool fiber. One hundred pounds of wool yield about 2 to 4 pounds of lanolin. Lanolin is recovered by wool-scouring, followed by separation and purification which may include acid cracking or centrifugal washing, neutralization, removal of soaps, filtration, bleaching and deodorization.
Lanolin is an effective emollient, which by subjective evaluation, effects softening and improvement of dry or rough skin caused by lack of sufficient natural moisture retention. Idson, B. ("What is a moisturizer?", Amer. Perf. Cosm. 87: 33-35 (August, 1972)) reported that lanolin causes the water in the skin to build up to its normal level of 10-30% by retarding, without completely inhibiting, trans-epidermal moisture loss.
A relative quantitative evaluation of the occlusive effect of lanolin and other cosmetic materials on the transpiration of moisture from human skin was made by Powers, et al. ("A study of the effect of cosmetic ingredients, creams and lotions on the rate of moisture loss from the skin", Proc. Sci. Sect. TGA, No. 28, 21-26 (December, 1957)). In particular, lanolin was applied to the inner surface of the forearm (5.0-6.25 mg/cm.sup.2, equivalent to a film thickness of 54-68 microns), and was covered with a 28 mm diameter glass desiccator containing silica gel. The uptake of water was determined by weighing the miniature desiccator after specific time intervals. All results were obtained under conditions of zero relative humidity. Lanolin caused a 32% reduction in moisture loss from the skin, indicating a mild occlusive effect for this material as opposed to the extreme barrier effect of petrolatum (48% reduction in moisture loss).
The moisturizing power of lanolin has been demonstrated an quantified by Clark, E. W. ("New concepts of lanolin", Manufacturing Chemist, 61:18-23 (July 1990)). When applied to human stratum corneum, lanolin has been shown to penetrate through the full depth down to the stratum lucidum (Clark, E. W. , "Short-term penetration of lanolin into human stratum corneum", J. Soc. Cosmet. Chem., 43-219-227 (July/Aug. 1992)). It has also been shown (in the "New concepts of lanolin" paper referred to above) that lanolin, when in contact with water, will spontaneously emulsify some of that water as a very finely dispersed water-in-oil (w/o) emulsion, with a droplet size less than 0.6 .mu.m, and, conversely, that water emulsified in lanolin can, if the internal/external humidity gradient is favorable, migrate out of the lanolin. In other words, lanolin permits the two-way transport of water and it is believed that lanolin within the stratum corneum can act as a moisture reservoir, absorbing natural epidermal moisture or releasing it depending upon the dryness state of the skin. Lanolin could thus behave in a way analogous to the natural intercellular lipid bilayers in human stratum corneum which hold water and control transepidermal water transport. As previously noted, an adequate moisture content within the stratum corneum is an essential requirement for good skin condition, and applications of lanolin assist in maintaining or restoring the required moisture level.
Lanolin may be used directly on the skin in its 100% form or be incorporated in mixtures or aqueous emulsions which may contain many other ingredients such as emulsifiers, oils, waxes, pigments, etc. Although lanolin exerts its maximum emollience when in the 100% form, the pure substance has a melting point above normal skin temperature and has a rather stiff consistency, which does not make for easy application to the skin, particularly sore skin. Thus, lanolin, per se, is not satisfactory as a skin treatment product because of its high viscosity, tackiness, and high drag property, thereby making it aesthetically unacceptable to consumers and too difficult to spread onto the skin to be widely accepted.
Accordingly, lanolin has historically been used as an auxiliary emulsifier in water-in-oil systems such as the traditional cold creams. In addition to performing as an auxiliary emulsifier, it improves the feel of oil and petrolatum base systems and imparts elegance and a silky smooth texture to the film on the skin. It also modifies the moisture permeability of the extremely occlusive hydrocarbons used in these systems and permits some diffusion of water vapor through the film. This property is related to the fact that lanolin contains a high concentration of hydroxy fatty acid esters of which about 80% are branched. In this respect it resembles human sebum and can in fact duplicate many of the functions of that substance when applied to the skin in cosmetic formulations.
In humans, the branched chain fractions of sebum are at their greatest in the skin of the fetus and diminish with age, becoming much reduced in adults principally because these fatty acids are not derived from metabolism but are by-products of the actual keratinization process, a process which decreases with age. Lanolin can therefore be though of as supplementing these reduced branched chain fatty acids and as a beneficial emollient, softening and super-fatting agent. Lanolin finds significant uses in lipstick and eye make-up preparations where its cosolvent and film modifying properties are exploited, almost certainly due to its complex branched ester composition. Typically, it reduces sweating or separation of components in pigmented systems. As might be expected, it is also very effective as a wetting and dispersing agent for pigmented medicaments (zinc oxide, calamine, etc.), and hydrocarbon-based ointments can be easily formulated if relatively large quantities of lanolin are included.
The derivatives of lanolin have also been extensively used as emollients. The derivatives of lanolin can be divided into two classes: physical derivatives, which are the result of physical separation processes such as fractional solvent crystallization; and chemical derivatives, which are the result of chemical modification of lanolin, per se, and hydrolysis of lanolin to fatty acids and fatty alcohols.
U.S. Patent No. 4,960,592 (Hagen, et al., "Lanolin And Lanolin Oil Skin Treatment Composition") seeks to overcome the aforementioned disadvantages of the use of lanolin, per se, by the utilization of a mixture of lanolin and lanolin oil. The lanolin oil, a physical derivative of lanolin (which consists mainly of various esters which cover a wide range of viscosity from mobile liquids to hard waxes), consists of those esters which are naturally liquid at room temperature, separated from whole lanolin by physical means. In particular, by mixing a proportion of lanolin oil with lanolin, the final composition has a reduced melting point, a reduced viscosity and a reduced drag on the skin. Apart from being different in physical consistency, lanolin oil is similar in chemical and general properties to whole lanolin and retains the high emollient power thereof. Although lanolin, and mixtures of lanolin with lanolin oil, are absorbed by the human stratum corneum, the rate of absorption is slow even when assisted by rubbing; and at loadings in excess of 2 mg/cm.sup.2 absorption is incomplete and a thin greasy film is left on the skin surface.
As such, a need continues to exist to an emollient composition which is effective and aesthetically acceptable.